CA1243317A - Thermographic system using naphthoylated leuco phenazine dyes - Google Patents

Abstract

Abstract of the Disclosure This invention involves thermographic imaging systems. The thermographic system under consideration comprises a leuco dye, a nitrate salt, and, optionally, an acid and stabilizing compound. Dyes in their reduced leuco form can provide the basis of color image forming systems. However, the leuco form of a given dye can often revert spontaneously to the colored form of the dye during manufacture unless the structure of the dye is altered to promote stability.
In this invention, which involves stabilization of the leuco dye, the stabilized leuco dye contains a phenazine nucleus with naphthoyl substituents on the 10-position and, optionally on one or both of the 3- and 7- positions through an amino group.
In their oxidized form, the leuco dyes of this invention provide access to a range of red and magenta colors which exhibit greater stability when subjected to light and temperatures characteristic of the stage of overhead projectors than that exhibited by previously available leuco dyes of this color range.

Description

33~7 -1- 60557-27~3 THERMOGRAPHIC SYSTEM USING
NAPHTHOYLATED LEUCO PHENAZINE DYES

BACKGROUND OF THE INVENTION
It is well known that dyes in -their reduced leuco form can provide the basis of color image forming systems. The leuco dyes may initially be relative]y colorless, hut can return to a colored form when oxidized, e.y., by air under acidic conditions.
Examples of leuco dyes used in color image forming systems include tria ylmethanes, xanthenes, styryl dyes, and azine dyes, such as, for example, phenoxazines, pheno-thiazines, and phenazines.
It is also known that the leuco form ofa ~iven dye can often revert spontaneously to the colored form of the dye, at the time of making -the leuco form, unless the structure of the dye is altered to promote stability.
In preparing transfer sheets and carbonless papers, where the leuco form of methylene blue [3,7-bis(dimethylamino) phenothiazine] has frequen-tly been used, stabilization has been brought about by acylation at the 10-position. For example, in U.S. Patent No. 2,646,367 benzoylated leuco methylene blue is disclosed for use in a carbonless paper system.
U.S. Pa-tent No. 2~783,227 discloses that benzoylated leuco methylene blue oE U.S. Patent No. 2,646,367, when used in carbonless papers, is not very light-stable, i.e. it decomposes to the blue-green color on exposure to light. Substitution of the l-naph-thoyl group or the 2-naphthoyl group in the 10~posi-tion provides more ligh-t-stable derivatives of leuco methyleneblue -than does 3~
-la- 60557-2743 benzoylation in the 10-position.
U.S. Pa-tent No. 2,783,228 discloses that nitrobenzoyla-ted leuco methylene blue is more light-stable than the benzoyla-ted leuco methylene blue.
The foregoing pa-tents concern acylated leuco `:~

methylene blue and explici~ly mention substitution with naphthoyl groups. They do not involve acylation in the 3,7-positions of phe~othiazine; -they involve acylation only of the ring nitrogen. Furthermore~ they make no distinction between the usefulness oE 1-naphthoyl groups and 2-naphthoyl groups.
German Offenlegunysscl~rift 2154659 discloses leuco dyes for use in carbonless papers, which dyes are derived from dye nuclei Oe phenothiazines, phenoxazines, and phenaæines by acylating the ring nitrogen wi-th a substituent Oe the Eorm V-Q-CO- where Q is O or NEI and V is an aromatic or aliphatic hydrocarbon. Nucleus substituents on the 3-position are secondary or tertiary amino groups, and subskituents on the 7~position are hydro~en, hydroxyl, amino, o~ substituted amino groups. Only the ring nitrogen is indicated as being acylated. German Offenlegungsschrift 2154660 discloses phenoxazines acylated on the ring nitrogen with a simpl* acyl radical ~pr~ferably ben~oyl).
Substituents on the 3- and 7-positions can he cho~en Ero~
h~droxyl, amino, or substituted amino groups, and one of the substituents on the 3- and 7-positions can be hydrogen.
The foregoing G~rman patents concern acylated azine leuco clyes including phenazines. However, these patents are not concerned with acylation of the unsubstituted amine groups in the 3- and 7-positions.
British Patent .SpeciEication 1,271,2~9 ~eals with ctabilized leuco dyes derived from water soluble dyes containing azine or oxazine rings in the chromophore. It discloses that the stabilized leuco dyes of the invention have an acyl radical attached to a tertiary nitrogen atorn in the reduced rin~ system, and that the acyl radical can be naphthoyl. However, this patent, like the foregoiny German patents, ignores the possibility of acylatiny the unsubstituted amine yroups in the 3- and 7-posltions.
Apart from their utility in transfer sheets ancl carbonless papers, acylated leuco Eorms of dyes, inclucling phenothia2ines and phenoxazines, have been Eound to be 3~

useful in thermographic imaging systems.
Japanese patent application JA 49-60264 da-ted May 30, 1974, and published as an examined application as JA52-23806 on June 27, 1977 discloses a bilayer thermographic color imaging system in which one layer comprises a colorless leuco dye with an inorganic nitrate under acid conditions. The leuco clyes are of -the phenothiazine and phenoxazine classes, acylatecd in the 10-posi-tion -to stabilize them. Only benzoyl, substitu-ted benzoyl, and alkoxy carbonyl groups are used, and the 3,7-amino substituents are limited -to tertiary amines.
Japanese patent application JA 49-67026 dated June 14, 1974, and published as an examined applica-tion as JA52-25330 on July 7,,1977 discloses leuco dyes having structural formulae dir-ected to phenoxazine and phenothiazine leuco dyes with acylation of the 10-position nitrogen. Although only acyl groups from fatty acids are included, naphthoyl leuco methylene blue is listed therein.
Current thermoyraphic systems employing combinations of leuco dyes and ni-trate sal-ts are exemplified by U.S. Patent No.
4,379,835 issued on April 12, 1933, which discloses a system eon-taining a minimum of two leuco dyes in reaetive assoeia-tion wi-th a nitra-te salt in a bincler, said system eapable of provicling a blaek image, and U.S. Patent No. 4,423,139 issued on Deeember 27, 1983, whieh diseloses a thermographic leuco dye/ni-trate salt system stabilized with a synergistic combination of an-tioxidants, e.g., phenidone plus ea-teehol. Amongs-t -the preferred elasses of dyes for the systems of U.S. Paten-t No. 4,379,835 and U.S. Pa-tent No.

: , , ... .

4,423,139 are N-acyl oxazine dyes, such as, for example, benzoy-lated leuco phenoxazine and N-acyl thiazine dyes, such as, f~r example, benzoylated leuco methylene b1ue.

i;~ - 3a -~, ~33~7~
-4- 60557-27~3 In previous systems of thermography based on combinations of leuco dyes and nitrate salts, at least two approaches have been applied to address the problem of stability to both thermal and actinic effects. The thermal effect, with respect to thermographic processes for preparing transparencies for overhead projectionr refers to the problem of attaining the highest possible thermal speed difference between temperatures ranging from 100C
to 200C, which range is typical of the image exposure, and those temperatures ranging from 25C to 60C, which range is typical of the projector stage after prolonged operation. The actinic effect refers to problems associated with the high light intensity and elevated temperature to which the finished transparency is subjected on the projector stage.
One approach involves adding to the imaging system stabilizing compounds, which, in most cases, are normally mild antioxidants, such as, for example, phenidone or ascorbic acid. A
second approach involves altering the structure of the leuco dye.
Although the first approach has sometimes been successful, it often leads to loss of sensitivity at imaging temperatures. Therefore, the second approach is preferable.
Although the previously mentioned, structurally altered leuco dyes are suitable for providing green and blue images, dyes for providing red and magenta images are so unstable that they require the addition of stabiliziny compounds. Even in the presence of stabilizing compounds, their stability i,s often marginal.
Examination of the prior art indicates that leuco dyes ~..

-~a~ 33~ 60557-2743 with a phenazine nualeus will provide image colors in the red region. These leuco dyes have been found to exhibit good stability when acylated in the 10-position. Further analysis indicates that a given leuco phenazine wi-th no acyl substituent in the 10-position is not only unstable to light, but is even much more unstable to light -than the analagous leuco pheno-thiazine which contains no acyl substl~uent in the 10 position. Although benzoyla-tion in the 10-position improves the light-stability of both leuco phenaæines and leuco phenothiazines, such benzoylated leuco phenothiazines are useful compounds Eor thermographic imaging compositionsl while the analagous benzoylated leuco phenazines are still far too unstable to be use~ul Eor similar types oE thermographic imaging compositions.

SVMMARY_OF THE INVENTION
This invention relates to thermal imaging sys-tems comprising stabilized leuco dyes in reactive association with nitrate salts in whi ~ the application of heat causes the reduced, relatively colorless form of the leuco dye to be oxidized to the colored dye Eorm. 3-Amino or 3,7-diamino phenazine leuco dyes are provided with naph-thoyl substituents in at least the 10-position and option-ally on one or both amino groups. l-Naphthoylation of such leuco phenazine dyes in the 10~position results in stabilities about an ord~r of magnitude better than that of benzoylated or other acylated Eorms of th~ dyes~ l~Naph-thoylation provides appreciably higher photo ~emical stabilit~ than d~s ~n~hth~y~ n~ whi~h, lta~l~O
provides higher s~ability -than does benzoylation. AS used in this application, the term "l~naphthoylationi' means acy-lation with a l-naphthoyl radical; the term ~ naphthoy~
lation" means acylation with a 2-naphthoyl radical~
Unsubstitut~d 3-amino and 7 amino groups can also be naphthoylated~ resulting in additional advantages, su~ as, Eor example, improvement in control of wavelength Oe the absorption peak of the oxidized leuco dye. The dyes retain good thermal imaging speed ~hile continuing to exhibit high stability to both thermal and actinic ef~ects brought about by ambient and overhead projection conditions. The dyes cover a wide range of visible colors when oxidized, including reds, yellows, and magentas~ Maphthoyl leuco phenaæines are suEficiently light stable to provide usahle stable red and magenta imaging dyes for the thermal imaging systems contemplated in this invention.

~;
DETAILED DESCRIPTION
In ~he thermographic systems of this invention comprising a leuco dye, capable of being oxidized to give a change in color, in reactive association with a nitrate salt, wherein the ca-~ion does not react with the leuco dye~
the leuco dye is a reduced form of a dye with a phenazine nucleus. As used herein, the term "change in color"
includes (1) chanc~e from an uncolored or lightly colored state (optical density less than 0.2) to a colored state (an increase in optical densi-ty of at least 0.2 units), and

(2) substantial change in hue. The structure of the reduced form of the dye is represented by the following general formula-0~ ~R2 Rl ~6 wherein Rl is selected ~rom the group consisting of alkyl radicals, aryl radicals, alkoxy radicals, and aroyl radicals, ~2 is selected from the group consisting of l-na~hthyl an~ 2-naphthyl, R3 and R4 are independently selected Erom the group consisting of hydrogen t haloyens, suhstituted or unsubstituted alkyl radicals, and aromatic rings condensed on the 1,2 and 8,9 positions, Z is selected Erom the group consisting oE
hydrogen, -OH, -SH, substituted or unsubstituted alkyl radicals, alkoxy radicals, -NR5R6, and -NR7R~r R5 and R6 are independently selected from the group consisting of hydrogen~ alkyl radicals, aryl ra~3icals, aralkyl radicals, 1-naphthoy1 radical, and 2-naphthoyl radical, provided that if either R5 or R6 is hydrogen, the other is not hydrogen,

3 3L~

R7 an~ R8 are independently selected from -the group consisting of hydrogen, alkyl radicals, aryl radicals, and aralykyl radicals, provided tha-t if either R7 or R8 is hydrogen, the other is not hydrogen.
If Rl is an alkyl radical, it is preEerred that it contain 1 to 10 carbon atoms. If Rl is an aryl radical, it is preferred that it only contain up to three condensed rings. If R3, R4, Z, R5, R6, R7, or R~ is an al]tyl radical, it is preferred that it contain 1 ~o ~ carbon atoms. When R3, R4 or Z is an alkyl radical, it can contain various different subs-tituents, so long as they are inert to the thermographic system and are not injurious to quality.
Such substituents can include, but are not limited to halogen such as chloride, bromide, fluoride, and iodide, hydroxy, and alkoxy~ and the like.
Leuco dyes that are suitable for the thermal imac3ing systems described herein include:
max of oxidized form, nm ~
O=C~>
¦ in acetone 575 ~ ~N ,~ 542 (C2~15~2N N ~ Nl-lOC ~ 442 O-IC in ethanol 577 " ~,N~" ~ 5~3 (C2~5~2N ~ l ~ 446 <O~
III O=C~

N ~7~N

IV O=C

N~N~ N

V O=C~>
in ~thanol 576 (C~2H5)2N N(C2H5)2 ~ ~3~

[~

N-C~

VII o=f~

8 C~13 N~C~'3 8 in eth~nol ~147 ~, VIII ~

~C~3)2~1 I_C~
~ '.

~ o -Nitrate salts suitable for thls invention are themselves well known. They may be supplied as various chemical compo~nds, but are desirably provided as a metal salt, and most preferably provided as a hydrated metal salt. Most means oE supplying the nitrate salt into the imagincJ composition are satlsfactory. For example, organic salts, metal salts, acid saltsr mixtures of acids and salts, and other means of supplying the iO;l are useful.
~itrates of zinc, cadmium, potassium, calcium, zirconyl (ZrO2), nickel, aluminum, chromium, iron, cc>pper, tin, magnesium, lead, and cobalt, ammonium nitrate, and cerous ammonium ~itrate can be used.
The nitrate salt component oE the present invention must be present in a ~orm within the imaging c~mpositlon so that oxidizing quantities oE ~IN03, N0, N02, or N204 will be provided wi-thin the composition when it is heated to a temperature no greater than 200C Eor 60 seconds and preferably no ~reater that 160C for 60 or most preEerably 30 seconds. The salt must be chosen so ~hat the cation thereo~ i5 non-reactive with the leuco dye.
~on-reactive salt~ are defined ln the practice of the present invention as those salts the cations of which do not spontaneously oxidi~e the dyes that they are ax~ociated with at rooln tempera-ture.
Preferred salts are the hydrated metal salts such as nickel nitrate hexahydrate, magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, ferric nitrate nonahydrate, cupric nitrate trihydrate, zinc nitrate hexahydrate, cadmium nitrate tetrahydrate, bismuth nitrate pentahydrate, thorium nitrate tetrahydrate, cobalt nitrate hexahydrate, gadolinium or lanthanum nitra~e nonahydrate, and mixtures oE these hydratecl nitrates. Nonhydrated or or~anic nitrates may be admixecl therewith.
It is preEerred to llave at least 0.10 mole of nitrate ion per mole of dye, It is more pre~erre~ to have at least 0,30 or 0.50 mole of nitrate ion per mole of dye.

3:~

The thermally s-timulated oxidation of the leuco dye by the nitrate salt can be facilitated by the presence of an acid. The acids optionally useEul in the thermographic system of this invention are aci~s as generally known to the skilled chemist. Organic acids, preferably those having carboxylic groups, su ~ as phthalic acid, are preferred, but irlorganic acids can also be used.
The acid can be present in a ratio of from 0 to 10 times the amount of the nitrate ~on.
The leuco dye, nitrate salt, and acid, when employed, are dissolved in a binder, whi ~ binder is neither strongly basic nor strongly acidic but which is sufficiently polar to hold the constituents in solution.
It is preferred that the binder be selected Erom polymeric materials. Such resins as polyvinyl acetals, e.g. poly-vinyl butyral, polyvinyl resins, polyvinylpyrrolidone, polyes~ers, polycarbonates, polyamides, polyacrylates, cellulos~ esters, copolymers and blends of these cl~sses oF
resins, can be used. Saran, a vinyl cllloride-vinylidene chloride copolymer, is particularly pre~erred. Natural polymeric materials su ~ as qelatin and ~um arahic can also be used.
The leuco dye should be present as at least 0.3%
by weight oE the binder, preEerably as at least 1~ by weight o the binder, and most preferably as from 2 to 10%
or more by weight of -the binder.
The preparation of the naphthoylated leuco dyes suitahle -Eor this invention follow procedures well known in the art. U.S. Patent No. ~l909r520 discloses a process for preparing acylated leucomethylene blue wherein the starting compound, a phenazine dye, is reduced with a suitable reducing agent, e.g., sodium hydrosulfite or sodium dithionite, under aqueou~ alkaline conditions in the presence of a water immiscible solvent capable of dissolving the leuco dye, e.g., methylene chloride or toluene. Naphthoyl chloride is then added to the reaction mixture, and the pH is adjusted to between 3 and 6, preferably about 4. AEter the reaction has proceeded for several hours, the pll i5 ra_sed to about 10, and the --.

33~'7 -12- 60557-27~3 methylene chloride layer, which contains -the leuco dye product, is separated. The naphthoylated leuco dye can be isolated from the solution by conven-tional separation techniques, including repeated decolorizing steps with active clays, such as, for example, attapulgus clay r or ac-tive carbon, recrystallizating from alcohol, or column chroma-toyraphy on silica gel.
The process of U.S. Patent No. 2,909,520 is useful not only for naphthoylating the lO-position of -the leuco dye, but also for naphthoylating unsubstituted amino groups, such as, for example, those in the 3- or 7- position or both. The process of that patent will result in the substitution of one of -the hydrogen atoms in the amino group. Although not markedly improving the stability of the leuco dye further, naphthoylation of an unsub-stituted amino group provides a further control of the wavelength of the absorp-tion peak of the oxidized leuco dye compared with the starting dye, i.e., the dye before reduction to the leuco form.
IR spectral investiga-tions show a disappearance of the doublet due to the pair of hydrogens on the amino groups and the NMR
spectra and molecular weights of the leuco dyes are consistent with the extra naphthoyl group or groups being present.
A formulation which can be applied by conventional coating techniques can be produced by dissolving the stabilized leuco dye, -the me-tal nitrate,~and the polymeric binder, together with an organic acid, and, optionally, a stabilizing compound, in an inert organic solvent, such as, for example, acetone, methyl ethyl ketone, or tetrahydrofuran.

xr '~.
i~ 1". ' f~s~ 33~
-12a- 60557-2743 The formulation can be coated onto a support by methods well known in the art, such as, for example, wire-wound rod, knife, or extrusion coating. Typical wet thiekness of the layer can range from abou-t 10 -to about 100 mierometers (~m), and the layer can be dried in forced air at -temperatures ranging from 20C to 50~C. It is preferred that the coating thickness be .elected to provide maY.imum .,~

-13~
ima~e densi-ties greater than 0.2, and more preEerably in the range 0.5 to 1.5, as neasured on a MacBeth Color Densitorneter Model T~ 504 using the color filter complementary to the dye color.
The support material can be selected from a wide range of materials~ including paper, glass, polymeric film, and the like, depending upon the particular ima~ing requirement. PreEerred materials include polymers having good heat stability, such as polyesters. A particularly preEerred polyester is polyethylene terephthalate.
The naphthoylated leuco phenazine dyes oE this invention provide several valuab'e properties, includin~, for example:
a) capability of changing color upon lS application of heat in thermal imaging systems based on oxidation by metal nitrate, b) improvement in stability to projection conditions, c) provision oE colors of the red, yellow and ma~enta hue, said colors previously ~einc3 available only with the trade-off of very low stability to heat and radiation.
With respect to stability to projection conditions, phenazine substituted with a naphthonyl group in the 10-position is better than phenazine substituted with a benzoyl group in the 10-position, and the l-naphthoyl species is better than the 2-naphthoyl species.
On oxidation in the thermographic sys-tem, the naphthoylated leuco phenazines lose the naphthoyl radical from the 10-position. However, no other naphthoyl radicals are lost, neither those on 3,7-amino ~roups nor the one in the 5-position in structur~ VIII. As stated previously, retention of these naphthoyl radical~s is important ~ecause they influence the light absorption characteristics oE the oxidized leuco dye.
The coatings prepared erom the leuco dyes Oe this invention were tested for stability by exposing ~ilrns con-~ ,, 33~ ~
taining imayed and unimayed areas on the staye of an over-head projector with the lamp switched on. Whereas ben70yl derivatives will generally exhibit unacceptable background density aEter a period of 3 to 4 hours, naphthoyl deriva-tives of the same leuco dyes will reach a similar unaccept-able condition only after about 15 hours. Increase in back-ground density is brought about mainly actinic effects, hut thermal effects also contribute somewhat.
Preparations 1-5 demonstrate the method for ~0 preparing naphthoylated leuco dyes of structures I, III, V, VI, VIl.

Preparation 1 The dye diethyl~henosafranine (0.05 mole) along with 1.0 g o~ ethylenediaminetetraacetic acid (EDTA) were dissolved in 1 liter of deionized water, and the solution was stirred thoroughly in an inert atmosphere (argon). The pH of the solution was about 3.5. Methylene chloride (500 mL) was then added, and the pH was adjusted to 10.0 by the addition of 25~ aqueous sodium hydroxide solution.
~odium dithionite (16 g) was then added, and the l~ll dropped rapidly to about 3. l-Naphthoyl chloride (0.15 mole) dissolved in 100 mL of methylene chloride was then adde~
alon~ with suficient sodium hydroxide to maintain the pH
between 3 and 4. The resulting mixture was stirretl Eor about 5 hours, during which time the pH rose to about 5.5.
The p~l was adiu~te~ t~ ln.~ w~th ~c~it~nal ~o~i~m hydroxide solution, and the resulting solution was stirred vigorously Eor one additional hour. The methylene chloride layer was then separated~ and ~he remaining aqueous layer extract~d twice with methylene chloride. This layer was then comhined with the oriyinal separated methylene chloride layer, To this combined volume was addecl 4 tablespoons of refinecl attapulyus clay (Attagel~ 50~ and magnesium sulEate to efEect drying, and the mixture was allowed to stand overnight in a separating Eunnel. I'he solids were then collectec~ by Eiltration and washed with~

methylene chloride. The combined liquid was decolorized two additional times with Attagel~ 50. The solvent, i.e., ~ethylene chloride, was then stripped o-~f under vacuu~, leavin~ a yellow powder. rhis powcler was purified several times by dissolving in ethyl alcohol with vi~orous stirrin~, filtering the remaining solid, and stripping the alcohol from the Eiltrate. The resultin~ product was the leuco dye of Structure I.

Preparation 2 The leuco dye oE Structure III was prepared by the procedure used in Preparation 1, the only exception being that phenosafranine was used in place of diethylphenosafranine.
Preparation 3 The leuco dye of Structure V was prepared by the proceclure used in Preparation 1, the only exception being that amethyst violet was used in place oE
~iethylphenosaEranine Preparation 4 The leuco dye of Structure VI was prepared by the procedure used in Preparation 1, the only exception bein~
that Magdala Red ~as usecl in place of diel:hylQllenosaEranine.

Preparation 5 The leuco dye of Structure VII was preparecl by the procedure used in Preparation 1, the only exception bein~ that Safranine-O was used in place oE
cliethylphenosafranine.

Preparation 6 The leuco dye of ,Ctructure II was prepared by the proceclure of Preparation 1, the only exception hein~ that 2-naphthoyl chloride was used in place oE l-naphtho~l chloride ~

Preparation 7 The leuco dye of Structure IV was prepared by the procedure of Preparation 2, -the only exception being that 2-naph-thoyl chloride was used in place of l-naphthoyl chloride.

Preparation 8 The leuco dye of Structure VIII was prepared by the procedure of Preparation 1, the only exception being that Neu-tral Red was used in place of diethylphenosafranine.

Preparations ~-E
Benzoylated counterparts of the leuco dyes represented by Structures I, III, V, VII and VIII were prepared by the procedures of Preparations 1, 2, 3, 5, and 8 respectively, the only exception being that benzoyl chloride was used in place of l-naphthoyl chloride. The benzoylated counterparts of leuco dyes I, III, V, VII and VIII are hereinafter referred to as leuco dyes IX, X, XI~ XII, and XIII, respectively.

Preparation F
A leuco dye was prepared according to the process of Preparation 1, the only excep-tion being that biphenyl carbamoyl chloride was employed as the acylating compound. The leuco dye produced is hereinaf-ter referred to as leuco dye XIV.
Exposure of coated films for providing test images was carried out on a platen heated -to 40C which had vacuum hold-down to ensure good contact between the film and the platen. The 33~
-17- 60557-27~5 film was then illuminated with a 1350 W infarared linear Eilament lamp with reflector situated 2.5 cm from the film. A
cam drive traversed -the film in front of the lamp a-t a continuously varying speed, thus producing exposure varying con-tinuously with distance along -the film.
Two models of 3M overhead projector were used in conducting the previously men-tioned image stabili-ty -tes-ts. Model 66 had a high wattage bulh and gave a rela-tively high s-tage tempera-ture. Model 213 hacl a lower wat-tage lamp together with a heat screen which gave a relatively lower stage temperature. These two projectors represent the range of stage conditions likely to be found in conventional usage.
The following examples serve to illustra-te the present invention and should not be deemed to be limitative thereof. All percen-tages are percent by weight unless otherwise indicated.
Example I

_ This Example demonstra-tes how images prepared from imaging compositions containing the leuco dyes of the present invention exhibi-t increased life on the stage of an overhead projector.
Formulations A and B were prepared by mixing the following ingredien-ts in the amoun-ts indicated:

~'.

3,3~
-17a- 60557-2743 A B-Ingredien-t Amount Amount Benzoyl leuco dye XI 0.07 g l-Naphthoyl leuco dye V 0.077 g 1% Phenidone in te-trahydrofuran 0.80 g 0.80 g 5% Ca-techol in te-trahydrofuran0.18 g 0.18 g Tetrahydrofuran 3.0 g 3.0 g 15% Methyl e-thyl ketone 8.0 g 8.0 g Formulation A and Formula-tion B were coated to a wet thickness of 3 mil (75~m) onto 4 mil (lOO~m) polyes-ter (TATS) and dried -for 5 minu-tes at 49C (120F). Both dried formulations were top-coated with a solution prepared from the following ingredien-ts in -the amounts indicated:

In~redient Amount Ni~No3)2 0.13 g p-Toluene sul~onic acid 0,075 g Wa~er 3.0 g 5% Polyvinyl alcohol 8.0 g ~Elvanol~ 52-22) in water Projector stage life results, i.e., background ima~e density versus time on the stage of a 3M ~odel 66 overhead projector, are shown in Table I. Background image densities were measured with a MacBeth Densitometer Model TD504O
Maximum image dye densi-ties of abo~lt 1.0 were used in all cases.

TABLE I
Backgro~nd_Ima~e_Den_ity v~ Time on _t~

Densitome~er ack~round ima~e density Time ~hrs) ~ilter A B
20 0 Yello~ 0.03 0,03 Red 0.02 0.02 (), O ~ O . 1 ~lu~ O . 0~ 0 . 0~1 25 1 Yellow 0.14 0.03 Red 0.03 0.02 Green 0.12 0.03 l~lue 0.07 0.0~

30 3 Yellow 0.39 0.14 Red 0.05 0.04 Green 0.39 0.13 ~lue 0.10 0.05 . ~
The increased stabilit~ o~ Formulation B cornpared with Fo~mulation A is clearly observable.

Example II
This Example demonstrates that l-naphthoylation of a leuco dye provides an image Eormed Erom an imaging composition containing the dye with ~reater stability than does 2-naphthoylation of ~he same leuco dye. Formulations C
and D were prepared by mixing the following ingre~ients in the amounts indicated:

C D
In~redient Amount Amount l-rlaphthoyl leuco Aye I 0.088 ~
2-Naphthoyl leuco dye XI 0.08a 9 Naphthoylate~ leuco dye 0.073 g 0.073 9 3,7-bis(diethylamino)-10 benæoyl-phenoxa.~ine 0.022 g 0.022 9 5~ Catechol in tetrahydrof~ran 0.12 g 0.012 1~ Phenidone in tetrahydrofuran 0.60 g 0.60 Phthalic acid 0.06 9 0.06 Tetrahydrofuran 5.0 g 5.0 Ni(No3)2 0.13 g 0.13 9 15~ Saranl in methyl ethyl Icetone ~.0 9 ~.0 FC-~131 (luorochemical nonionic surfactant available from 3M, St. Paul, MN) l drop ` l drop -1 Saran is a commercially available copolymer of vinyl chloride-vinylidene chloride.

~oth formulations were coated to a wet thickness oE ~ mil (75 ~m) on a 4 mil polyester film (100~ m) and drie~d a~ 52C
(125F) for 4.5 minutes. Stability results are shown in Table II.

, . , -20- ~33~
TABLE II
Back~round Ima~e Density vs Time on Stage Densitometer ~ack~round ima~e densit~
5Time (hrs) Eilter C D
Yellow 0.03 0.03 O Red 0.02 0.03 Green 0.03 0.03 81ue 0.04 0.0 Yellow 0.04 0.10 6 Red 0.04 0.05 Green 0.0~ 0.10 Blue 0.05 0-09 1 3M Model 66 Overhead Projector The increased stability of Formulation C compared wi~th Formulation D is clearly observable.

Example III
This Example demonstrates the enhancement of photochemical stability of an image brought about hy replacing a given ben~oyl phena~ine leuco dye o~ an ima~ing composition with its l-naphthoyl analo~. Formulations ~ and F were prepared by mixing the following ingredients in the amounts indicatedO

-21~ 3~
E F
In~redient_ _ _ __ ~moun~ Amount Leuco dye XV 0~023 g 00023 ~
Benzoyl leuco dye X 0.050 g 0.050 g 5 Benzoyl leuco dye IX 0.058 g 1 Naphthoyl leuco dye I 0.092 g 5% Catechol in tetrahydrofuran 0012 g 0.12 Ph~halic acid . 0.06 g 0.06 g 1~ Phenidone in tetrahydrofuran 0.65 g 0.65 g Ni(N~3)2 0.1~ g 0.12 g 15% Saran in methyl ethyl k~tone 8.6 g 8.0 Tetrahydro~uran 5.0 9 5.0 g FC 431 1 drop 1 drop Formulations ~ and F were coated to a wet thickness o~ 3 mils (75 ~m) onto 4 mil ~100 ~m) polyester (TATS) and drie~
for 5 minutes at ~9~C ~120F~. Stability results are shown in Table III.

~ABLE III

Time Densitometer ack~ro nd ima~e der,sit~
_ Eilter _ E F
O Yellow 0.03 0.03 Red 0.03 0.03 Green 0.03 0.03 Blue -~ 04 ~llow O.OB o.n~
Red -Q~ 0 03 Green 0.10 0.05 a:Lue 0.11 0.07 B Yellow 0.22 0.09 Red 0.06 0.05 Green 0.26 0.10 ~lue 0,2~ 0.12 3M Model 66 Overhead Projector.

~ 33~ ~

The increased stability of Formulation F compared ~7ith Formulation E is clearly observable.

Example IV
This Example demonstrates that photochemical stability of an image is further enhanced when both phenazine dyes o the imaging composition are present in the l-naphthoyl leuco forms. Formulakion G was prepared by mixing the ~ollowing in~redients in the amounts indicated:
G
Ingredient _ __ Amount ~euco dye XV 0.023 g 1-Naphthoyl leuco dye III 0,075 9 l-Naphthoyl leuco dye I 0.092 ~
5~ Catechol in tetrahydrofuran 0.075 g Phthalic acid 0.06 g 1~ Phenidone in tetrahydro~uran 0.70 g Ni~NO3)2 0.13 15~ Saran in methyl ethyl ketone 8.0 20 ~etrahydro~uran 5,0 g FC-431 1 drop Formulation ~ ~as coated to a wet thickness o~ 3 mils (75 ~m3 on a 4 mil (100 ~m) polyester ~ilm and c~rie~ a~ 49C
(120~F) for 5 minutes, Stability results are shown in Table IV.

3~

~L 3 ~ 7 _~3~
TABLE IV

~,~ensity vs Time on Sta~e3 Background Densitometerima~e density Time ~hrs) filter G
o Yellow 0.04 Red 0.03 Green 0.03 Blue n . 04 Yellow 0.03 Red 0.03 Green 0O03 ~luf~ 0. 0 13 Yellow 0,05 Red o.o~
Green 0.04 Blue 0,06 19 Yellow 0.07 Red 0-05 Green 0.07 Blue -~

_ _ _ _ _ , 3 3M Model 213 Overhead Projec~or Upon comparison with the results in Table III, it can be observed that a ~urther considerable improvement in stability has occurred by employing a second naphthoylated phenazine dye in the image Eorming system.

2~ 33~ ~
Example V
This Example illustrates the improvement in image stability resulting from the use oE naphthoylated leuco clye I in the imaging composition in place of the equivalent p-phenyl ben~oyl leuco dye XIV.
Formulations H and I were prepared by mixing the Eollowing ingredients in the amounts indicated:

H
Ingredient Amount Amount _. .. ~....... _ Pergascript turquoise 0.023 g 0.025 g Naphthoyl diazine yellow 0.07 g 0.07 g ~etrahydrofuran 5,0 g 5.0 1% Phenidone in tetrahydro~uran 0.6 g 0.6 g 5~ Catechol in tetrahydroE~ran 0.12 g 0,12 g Phtnalic acid 0.06 g 0,06 g Ni(NO3~2 0~14 g 0.14 15~ Saran in methyl ethyl ketone 8.0 ~ 8.0 g FC-431 1 drop 1 drop Leuco dye I o.og g Leuco dye XIV 0.09 Formulations l1 ~nd J were coated to a wet thickness of 3 mils ~75 ~m) on ~ mil (100 ~m~ polyester ~ilm and dried at 52C ~125F) Eor 5 minutes.
The coated ~ilms were tested on the Model 213 Overhead Projactor. The results are shown in Table V.

TA~LE V
~ = . _ Densitometer aclc~round_imac~e_clensit~
filter Film H4 Film H5 ... _ _ . . .
Yellow 0.07 0.12 Red n.06 0.07 35 Green 0 08 0.12 Blue 0.10 0.13 _ _ ~ . . _ . . . _

4 Time on stage was 13-1/2 hour~

5 Time on stage was 6 hours.

-25~ ~ 33~
The naphthoylated leuco dye shc~wed substan-tial advantage over the analagous p-phenyl ben~oylated leuco dye. This result demonstrates that the effectiveness of the naphthoyl radical is not due to its molecular weight being higher than that of the benzoyl radical. The p-phenyl henzoylated leuco dye exhibits stability more similar to that exhibited by the benzoylated leuco dye than to that exhlbitecl by the naphthoylated leuco dye.
Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and i~ should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a thermographic system comprising at least one leuco dye in reactive association with a nitrate salt, wherein the action of heat can oxidize said at least one leuco dye to give a change in color, the improvement wherein said at least one leuco dye consists of a phenazine dye in its reduced form having a structural formula wherein R1 is selected from the group consisting of alkyl radicals, aryl radicals, alkoxy radicals, and aroyl radicals, R2 is selected from the group consisting of 1-naphthyl and 2-naphthyl, R3 and R4 are independently selected from the group consisting of hydrogen, halogens, substituted or unsubstituted alkyl radicals, and aromatic rings condensed on the 1,2 and 8,9 positions, Z is selected from the group consisting of hydrogen, -OH, -SH, substituted or unsubstituted alkyl radicals, alkoxy radicals, -NR5R6, and -NR7R8, R5 and R6 are independently selected from the group consisting of hydrogen, alkyl radicals, aryl radicals, aralkyl radicals, 1-naphthoyl radical, and 2-naphthoyl radicals, provided that if either R5 or R6 is hydrogen, the other is not hydrogen, R7 and R8 are independently selected from the group consisting of hydrogen, alkyl radicals, aryl radicals, and aralkyl radicals, provided that if either R7 or R8 is hydrogen, the other is not hydrogen.

2, A thermographic system according to claim 1 wherein at least one of the said leuco dyes is selected from the group consisting of

3. A thermographic system according to claim 1 further including an acid.

4. A thermographic system according to claim 3 wherein said acid is an organic acid having at least one carboxylic group.

5. A thermographic system according to claim 1 further including a stabilizing compound selected from the group consisting of phenidone, catechol, and hydroquinone.

6. A thermographic system according to claim 1 further including saran.

7. A thermographic system according to claim 1 wherein at least 0.10 mole of nitrate ion is present per mole of dye.

8. A thermographic system according to claim 7 wherein at least 0.50 mole of nitrate ion is present per mole of dye.

9. A thermographic system according to claim 3 wherein the acid is present in a ratio of from 0 to 10 times the amount of nitrate ion.

10. A leuco phenazine dye having the structural formula wherein R1 is selected from the group consisting of alkyl radicals, aryl radicals, alkoxy radicals, and aroyl radicals, R2 is selected from the group consisting of 1-naphthyl and 2-naphthyl, R3 and R4 are independently selected from the group consisting of hydrogen, halogens, substituted or unsubstituted alkyl radicals, and aromatic rings condensed on the 1,2 and 8,9 positions, Z is selected from the group consisting of hydrgen, -OH,-SH, substituted or unsubstituted alkyl radicals, alkoxy radicals, -NR5R6, and -NR7R8, R5 and R6 are independently selected from the group consisting of hydrogen, alkyl radicals, aryl radicals, aralkyl radicals, 1-naphthoyl radical, and 2-naphthoyl radical, provided that if either R5 or R6 is hydrogen, the other is not hydroyen, R7 and R8 are independently selected from the group consisting of hydrogen, alkyl radicals, aryl radicals, and aralykyl radicals, provided that if either R7 or R8 is hydrogen, the other is not hydrogen.

11. A leuco phenazine dye having the structural formula wherein R1 is selected from the group consisting of alkyl radicals, having 1 to 10 carbon atoms, aryl radicals having up to three condensed rings, alkoxy radicals, and aroyl radicals, R2 is selected from the group consisting of 1-naphthyl and 2-naphthyl, R3 and R4 are independently selected from the group consisting of hydrogen, halogens, substituted or unsubstituted alkyl radicals having 1 to 4 carbon atoms, and aromatic rings condensed on the 1,2 and 8,9 positions, Z is selected from the group consisting of hydrogen, -OH, -SH, substituted or unsubstituted alkyl radicals having 1 to 4 carbon atoms, alkoxy radicals, -NR5R6, and -NR7R8, R5 and R6 are independently selected from the group consisting of hydrogen, alkyl radicals having 1 to 4 carbon atoms, aryl radicals, aralkyl radicals, 1-naphthoyl radical, and 2-naphthoyl radical, pro-vided that if either R5 or R6 is hydrogen, the other is not hydrogen, R7 and R8 are independently selected from the group consisting of hydrogen, alkyl radicals, aryl radicals having 1 to 4 carbon atoms, and aralykyl radicals, provided that if either R7 or R8 is hydrogen, the other is not hydrogen.

12. A leuco phenazine dye selected from the group consisting of